ar71xx: configure OBS4 line on TL-WR841N-v8/MR3420v2

It also fixes USB Power on MR3420v2.

This time we took the information from the source of GPL:
http://www.mail-archive.com/openwrt-devel@lists.openwrt.org/msg18970.html
Confirmed and tested:
https://dev.openwrt.org/ticket/13201#comment:41
Thanks to oguretsagressive for testing.

Signed-off-by: Dmytro <dioptimizer@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt@37878 3c298f89-4303-0410-b956-a3cf2f4a3e73